Shafts of ancient ice extracted from Antarctica's frozen depths show that for at least 650,000 years three important heat-trapping greenhouse gases never reached recent atmospheric levels caused by human activities, scientists are reporting today.
|Drilling tower at Dome Concordia. Photo © A. Fornet/IFRTP - The single most important source of information about past climate change and the associated composition of the atmosphere are the two large ice caps of Greenland and Antarctica. Analysis of ice cores is therefore the most powerful means we have to determine how climate has changed over the last few climate cycles, and to relate this to changes in atmospheric composition, in particular to concentrations of the principal greenhouse gases - CO2, CH4 and N2O (carbon dioxide, methane, and nitrous oxide). |
In an article in the journal Science titled Tiny Bubbles Tell All, Edward J. Brook says that during the past 200 years, humans have caused a remarkable change in the levels of several atmospheric greenhouse gases. We know this from direct measurements that started in the latter half of the 20th century, but for earlier times we rely on tiny samples of the atmosphere trapped in polar ice. Coring the polar ice sheets provides access to these samples and allows us to place modern changes in the context of long-term natural cycles in greenhouse gases. Until recently, the longest of these ice core records (from the Russian research station Vostok Station in Antarctica) extended back 440,000 years. Now, the window into the past has been extended an additional 210,000 years.
Since 1997, the oldest ice available for analysis was that from the Vostok, Antarctica, ice core, which extends back to 420,000 years ago and covers four complete glacial cycles. A new ice core from the European Program for Ice Coring in Antarctica, (EPICA) Dome C site in Antarctica now extends back to an age of 740,000 years or more. Two reports present data on the composition of the atmosphere between 400,000 and 650,000 years ago, an interval soon after glacial cycles switched from a dominantly 41,000-year period to the dominantly 100,000-year period that occurs today.
Brook says that two basic messages are apparent in this extended history of the atmosphere. First, even with this longer perspective, the modern atmosphere is still highly anomalous. At no time in the past 650,000 years is there evidence for levels of carbon dioxide or methane significantly higher than values just before the Industrial Revolution. Second, the covariation of carbon dioxide and methane with climate, strikingly evident in the Vostok record, follows essentially the same pattern in the earlier time period. The muted climate cycles are accompanied by equally muted cycles of carbon dioxide and methane (see the figure). This relationship reinforces the view that the large-scale cycles in Antarctic temperature have global importance, and that climate and greenhouse gas cycles are intimately related.
|The long view.The greenhouse gas (CO2,CH4, and NO2) and deuterium (À D) records for the past 650,000 years from EPICA Dome C and other ice cores,with marine isotope stage correlations (labeled at lower right) for stages 11 to 16 (2, 3). À D, a proxy for air temperature, is the deuterium/hydrogen ratio of the ice, expressed as a per mil deviation from the value of an isotope standard (4). More positive values indicate warmer conditions. Data for the past 200 years from other ice core records (20–22) and direct atmospheric measurements at the South Pole (23, 24) are also included. Source: Science|
Levels of carbon dioxide have risen from 280 parts per million two centuries ago to 380 ppm today. Earth's average temperature, meanwhile, rose about 1 degree Fahrenheit in recent decades, a relatively rapid rise. Many climate specialists warn that continued warming could have severe effects, such as rising sea levels and changing rainfall patterns.
Brook says for nitrous oxide, the picture is slightly less clear. The record is not complete, making it difficult to judge how or whether the amplitude of 100,000-year cycles changed with time, and anomalous levels of nitrous oxide appear to be related to high levels of dust in the ice.
Skeptics often dismiss the rise in greenhouse gases as part of a naturally fluctuating cycle. The new study provides ever-more definitive evidence countering that view.
Today's rising level of carbon dioxide already is 27 percent higher than its peak during all those millennia, said lead researcher Thomas Stocker of the University of Bern, Switzerland.
"We are out of that natural range today," he said.
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